Surface finishing processes for metal deposition in the manufacture of printed circuit boards, IC substrates, microchips and related electronic devices are employed to provide bondable and/or solderable surface areas whereon active and passive components can be mechanically and electrically contacted to electronic devices. Frequently applied surface finishing processes are the ENIG process (Electroless Nickel Immersion Gold) and Ni/Pd(/Au) processes. In both types of processes, a copper surface of an electronic device is activated with an activation composition containing noble metal ions followed by electroless (autocatalytic) deposition of a metal or metal alloy layer thereon.
An activation of a copper surface prior to electroless plating of e.g. nickel is required because copper is classified as a non-catalytic metal for electroless deposition of for example nickel deposited from hypophosphite containing electrolytes (W. Riedel, Electroless Nickel Plating, ASM International, reprint 1998, p. 189). The purpose of the activation step is to deposit an immersion coating of a catalyst on the copper surface. The catalyst lowers the activation energy and allows a metal such as nickel or a metal alloy such as a nickel-phosphorous alloy to be deposited on the copper surface by electroless plating. Examples of suitable catalysts are palladium and ruthenium (Printed Circuits Handbook, Ed.: C. F. Coombs, Jr., McGraw-Hill, 2001, p. 32.3). Usually, a palladium seed layer is immersion plated onto the copper surface, wherein the palladium ions are deposited onto the copper surface, reduced by copper to metallic state palladium and copper ions are released into the immersion plating bath. The copper surface is then coated with metallic palladium which serves as a seed/catalyst for the following electroless metal or metal alloy plating.
It is important to rinse the substrate carefully between activation of the copper surface with noble metal ions and electroless metal or metal alloy plating to remove all excessive noble metal ions. Otherwise precipitates formed of e.g. noble metal hydroxides can lead to extraneous (uncontrolled) metal or metal alloy deposition around individual copper features on the substrate surface, surfaces made of plastic materials and other dielectric materials.
The phrase ‘plastic materials’ comprises bare PCB laminates, solder masks and photoimageable resists such as dry film resists. Other materials which can be affected by said uncontrolled deposition of noble metal ions or precipitates thereof comprise silicon dioxide, silicon and non-metallic glasses.
The noble metal ion precipitates act as seed/catalyst for uncontrolled and undesired deposition of a metal or metal alloy during electroless plating. Typical phenomena of extraneous metal or metal alloy plating are for example nickel bridges and nickel feet formed during electroless plating of nickel. Extraneous plating leads for example to circuit shorts, especially in high density circuits with line and space widths of 50 μm or less. Extraneous metal or metal alloy deposits have a weak adhesion on laminate and solder mask surfaces and can fracture off the PCB and then also cause circuit shorts at other locations of the PCB. Metal and metal alloy bridges between individual copper contact pads or copper trenches can directly result in circuit shorts. Metal and metal alloy feet around copper contact pads in fine line circuits also can cause circuit shorts by bridging individual copper contact pads.
The problem of extraneous plating, particularly of nickel is known in the prior art and different approaches towards this problem are described:
US 2001/0040047 teaches a method which reduces bridging that can occur during plating. Said method comprises the following steps: contacting a circuitized substrate with a swelling agent, treating the substrate with a composition of an alkaline permanganate, a chromate or chlorite salt followed by applying a metal layer on the treated circuitized portion of the substrate. The method is an alternative to the process according to the present invention as no noble metal ion activation is applied.
US 2008/0073614 A1 discloses an acidic etchant for metallic palladium comprising at least 500 mg/l thiourea. Such an etchant is not suitable to selectively remove palladium ions or precipitates thereof and results in undesired skip plating (Example 5).
The patent document EP 0 707 093 discloses an activator which will selectively activate the copper surfaces for electroless nickel plating and thereby minimize or eliminate extraneous plating. Said activator composition comprises an imidazole compound and may further contain palladium ions.
Excess noble metal ions during activation of a copper surface tend to hydrolyze in the rinse step(s) applied before electroless plating of nickel and form noble metal-containing precipitates. Said precipitates can be adsorbed onto surface areas of the substrate that are made of plastic or other dielectric materials or can be trapped in cavities formed between structural features made of copper and plastic or other dielectric materials, e.g., laminates, photoresists and/or solder masks on the substrate surface. Said precipitates can be reduced to metallic state noble metal by the reducing agent present in the electroless metal or metal alloy plating bath utilized for depositing the metal or metal alloy layer onto the substrate (step iv. of the process according to the present invention). Metal or metal alloy plating will occur on such reduced noble metal precipitates. If the reduced noble metal is present on surface areas of the substrate other than the copper surface, unwanted extraneous metal or metal alloy plating can occur and lead to undesired circuit shorts.
A rinse step is applied after activation with noble metal ions in order to remove all excess noble metal ions before contacting the substrate with the electroless metal or metal alloy plating composition. On the other hand, hydrolysis and precipitation of noble metal ions occurs preferably during such rinse steps. Therefore, at least one rinse step is usually done in a rinse solution consisting of diluted sulfuric acid. But in case of high density circuits the sulfuric acid rinse is not sufficient to suppress extraneous metal or metal alloy plating.
Another method to suppress undesired extraneous metal and metal alloy plating is disclosed in EP 2233608 A1. The substrate is contacted with an aqueous composition comprising an organic aminocarboxylic acid after activation of the substrate with noble metal ions and prior to electroless nickel plating.
A method for depositing a nickel alloy onto a metal substrate by electroless plating is disclosed in U.S. Pat. No. 5,843,538 A. The substrate is contacted, in this order, with an aqueous solution comprising an organic acid and a fluorosurfactant, then with an activator solution comprising palladium ions and then with an electroless nickel plating bath which comprises thiourea as a stabilizer additive. Extraneous plating is not suppressed by this method.
A palladium removal treatment is disclosed in US 2013/005862 A1. A copper surface is contacted after activation with palladium ions with an alkaline solution comprising a sulfur compound. Such alkaline solutions suppress extraneous plating but result in undesired skip plating of the copper surface, i.e. palladium is also removed from the copper surface where it should remain in order to initiate electroless nickel plating (Examples 9 and 10).
An etchant for metals such as palladium comprising hydrochloric acid and thiourea is disclosed in US 2005/0109734 A1. This etchant is applied to remove oxides from the surface of metals such as palladium prior to etching of metallic palladium. Accordingly, said etchant is not suitable to selectively remove metallic palladium from a substrate without causing undesired skip plating.
In addition, palladium ions or precipitates formed thereof are adsorbed onto the copper contact pads and should not be removed before electroless plating in order to prevent skip plating. Skip plating is an undesired phenomenon wherein an incomplete metal or metal plating occurs on the copper contact pads. Accordingly, such skip plating results in a disrupted metal or metal alloy layer on top of the copper contact pads and leads to failures during bonding and soldering.